Supported cobalt catalysts wherein the cobalt is in its elemental or reduced state are well known and find use in many reactions involving hydrogen such as hydrogenation reactions, and the Fischer-Tropsch synthesis of hydrocarbons. The activity of the catalysts is believed to be directly proportional to the cobalt surface area of the reduced catalysts, but in order to achieve high cobalt surface areas, the cobalt should be well dispersed on the support. Furthermore, to minimize reactor volume, the catalyst should preferably have as high a cobalt content as possible. High cobalt contents also offer improved efficiency in catalyst recycle and regeneration. However, as the cobalt content of a catalyst increases above 20% by weight (on reduced catalyst) the cobalt becomes more difficult to disperse resulting in lower cobalt surface areas. Cobalt is a relatively expensive metal and therefore there is a desire to improve the cobalt dispersion (expressed as cobalt surface area per gram catalyst).
Preparation of cobalt-alumina catalysts has heretofore typically been by impregnation of cobalt compounds into ‘pre-formed’ alumina materials or by precipitation of cobalt compounds from solution in the presence of alumina powders or extrudates, followed usually by a heating step and then, prior to use, reduction of the resulting cobalt compounds to elemental form using hydrogen.
Alternatively cobalt-alumina catalysts may be prepared by simultaneous co-precipitation of cobalt (Co) and aluminium (Al) compounds by addition of a base. Khassin et al describe the co-precipitation of Co2+ and Al3+ at ratios of 1:1, 1:1.3 and 1:2 to yield hydrotalcite-containing materials. Upon calcination and reduction under hydrogen at 470-480° C. these catalysts were of low activity in the Fischer-Tropsch synthesis of hydrocarbons (see Journal of Molecular Catalysis A: Chemical, 168, 2001, 193-207).